In recent years, in the field of mobile communications, not only mobile terminals but also base stations are required to be made smaller and lighter. In this respect, since the power amplifier sections of relevant equipment greatly affect the sizes and weights of the equipment, it is important to improve efficiency of the power amplifier sections. Mobile terminals, in particular, are normally powered by batteries, so that the length of time during which they are continuously operable is limited. In the circuit of a mobile terminal, its power amplifier section consumes the most power. Reducing the power consumption of the power amplifier section (that is, making the power amplifier section more efficient) is therefore a major task to be solved to make the mobile terminal continuously operable for an extended period of time. For a base station, too, to allow its equipment to be installed close to an antenna so as to reduce cable loss, making its high-frequency power amplifier section smaller and lighter is required. Hence, improving efficiency of the high-frequency power amplifier section is essential.
Generally, the efficiency of a high-frequency power amplifier using a semiconductor device increases with its output level, achieving maximum efficiency around where the output is saturated. The maximum output (saturation power) level obtainable from a high-frequency power amplifier depends on the size of a semiconductor device used in it. If, to improve the efficiency in a low output power range of a high-frequency power amplifier, the semiconductor device included in the high-frequency power amplifier is made smaller, causing its output saturation level to be lowered, the high-frequency power amplifier will become unable to provide a high output power when required. If a high-frequency power amplifier is arranged to render high efficiency in a high output power range, its efficiency will remarkably deteriorate in a low output power range. Thus, it is extremely difficult to make a high-frequency power amplifier highly efficient both in a high output power range and in a low output power range.
A known arrangement for achieving highly efficient operation both in a high output power range and in a low output power range includes plural output stage amplifiers and switches. In the arrangement, the switches are used to selectively use the amplifiers according to the required output level (see the patent document 1, for example).
In another known arrangement intended to further improve efficiency, switching between plural output stage amplifiers is effected using no switches (see the patent document 2, for example).
[Patent Document 1] JP-A No. 07-336168
[Patent Document 2] JP-A No. 2003-046340